(a) Field of the Invention
The present invention relates to a method for maximizing medium access control (MAC) throughput by dynamically changing an RTS (Request To Send) threshold providing for a selective use of RTS-CTS (Clear To Send) frame exchange in an IEEE 802.11 wireless LAN system.
(b) Description of the Related Art
Each station STA in the IEEE 802.11 wireless LAN system implements an access point (AP) and a MAC layer. Before frame transmission, the MAC layer acquires access to the network in either a contention mode, i.e., a distribution coordination function (DCF) mode, or a non-contention mode, i.e., a point coordination function (PCF) mode.
The DCF provides an elementary access protocol for automatic sharing wireless media among the stations having compatible physical layers and the AP.
FIGS. 1a and 1b are timing diagrams showing the DCF operation mechanism.
FIG. 1a is a timing diagram of the two-step handshake type DCF operation mode.
While a station STA1 is using a wireless medium for packet transmission in the contention mode, another station STA2 has to wait for the use of the wireless medium.
Here, there is a standard access spacing that delays the access of the station to the shared wireless medium. Short interframe space (SIFS) is an access spacing for the shortest frame interval and the highest priority, and is typically used for ACK, CTS frame, etc. Distributed interframe space (DIFS) is a time interval for transmitting data and control frames in the DCF mode and is longer than the SIFS. Referring to FIG. 1a, the station STA2 tries to gain access to the medium after the access spacing and an elapse of a back-off time, only when it receives an acknowledge signal ACK for data reception from the station STA1.
FIG. 1b shows an RTS/CTS operation mechanism applied in the case where the size of the data frame exceeds a threshold (hereinafter, referred to as “RTS threshold”).
The RTS frame is a control frame transmitted to the destination station STA2 by the origination station STA1 for frame transmission. The RTS frame is transmitted exclusively for the data frame greater than the predefined RTS threshold.
The CTS frame is a frame transmitted by the destination station upon receiving the RTS frame, to indicate that the origination station is allowed to transmit the data frame. At this time, a third station STA3 does not try to gain access to the medium so as to avoid any confliction.
According to preceding research analyzing the capacity of the IEEE 802.11 DCF protocol, it is assumed for expedience that all the stations generate a data frame of a predetermined length, so the IEEE 802.11 DCF protocol cannot support a selective RTS-CTS frame exchange depending on the length of the data frame.
On a second assumption that the station in the BSS (Basic Service Set) has a greedy traffic generation profile in a theoretical capacity analysis, it is impossible to provide an accurate solution for determining the MAC throughput for a general traffic type.
According to the IEEE 802.11, the data frame, if equal to or smaller than a specific threshold (i.e., RTS threshold), is transmitted through a physical carrier sense without using RTS and CTS frames. Alternatively, the data frame, if greater than the RTS threshold, is transmitted after the RTS and CTS frames are transmitted.
This is for transmitting the RTS and CTS frames only for the transmission of a relatively large data frame in consideration of the traffic load affecting the wireless media during a retransmission so as to protect from inefficient use of the wireless media caused by successive transmission of the RTS and CTS frames.
Therefore, the use of the RTS/CTS frame must be taken into consideration in the capacity analysis of the wireless LAN system. In addition, the RTS threshold that is a threshold to determine whether to transmit the RTS/CTS frame in transmission of the data frame must be determined in consideration of the capacity of the wireless LAN system so as to maximize the capacity of the wireless LAN system.
As described above, however, the conventional techniques cannot provide a method for changing the RTS threshold to maximize the MAC throughput.